Electrostatic transducers for generating and detecting sound waves are well known in the art and attention is directed to U.S. Pat. Nos. 3,544,733; 3,946,422; 3,961,291; 4,329,547; 4,419,545; 4,908,805; 4,429,193; 4,439,642; 4,885,781; 4,993,072; 5,101,543; 4,809,355; 4,594,897; 4,85,783; 4,558,184 and 4,593,567, which are typical of known devices and which generally include a moveable conductive material in juxtaposition to a moveable non-conductive dielectric material, both of which vibrate in unison. In most instances a conductive backplate in the form of a plate or foil is spaced from the dielectric material which is usually a thin film of plastic such as a Mylar film. Between the two there is an air gap or space of predetermined thickness. The size and thickness of the air gap is critical and many attempts have been made to vary this gap by the use of multi dielectric layers, and spaces of varying types. The problem is that ultrasound is highly attenuative in air above about 600 kHz and conventional ultrasonic devices cannot adequately couple vibrational energy from solid to gas due to the large impedance mismatch. Use of conventional transducers for such applications as non-destructive testing of large planar planar surfaces is greatly hampered by the need to use an external coupling agent such as a gel. Many prior art patents rely on a simple air pocket between the dielectric material and the backing electrode so that the dielectric does not touch the backing electrode. Others, such as U.S. Pat. No. 4,908,805, support the dielectric membrane covering the backing electrode on posts and use glue to hold the membrane in place, and then provide openings in the backplate. They are designed for use at relatively low frequencies and the space taken up by the glue is effectively wasted space that could be used for generation of ultrasonic energy. U.S. Pat. No. 4,419,545 provides means for varying the area of the open holes in the backplate leading to the single air pocket in order to affect the frequency response and sensitivity but the vibrating element does not touch the back plate.
There is, therefore a need for improved acoustic transducers which can operate efficiently at frequencies up to at least 2 MHz, and it is believed that this can be attained by careful control of the surface roughness of the backplate so as to provide a plurality of air pockets each of which vibrates. Such transducers would find applications testing large surfaces for internal flaws, among other applications obvious to those skilled in the art. Such other applications include human diagnostic and therapeutic uses. As is well known ultrasound, using a gel coupling agent, is used extensively for medical purposes.